Maximum Depth of Binary Tree

题目：

Given a binary tree, find its maximum depth.
The maximum depth is the number of nodes along the longest path from the root node
down to the farthest leaf node.

解答：

public class Solution {

    public int maxDepth(TreeNode root) {
        if(root == null) {
            return 0;
        }

        return Math.max(maxDepth(root.left), maxDepth(root.right)) + 1;
    }
}